Interpretive Summary: Citrus seedlings are slow to begin flowering and fruiting which delays evaluation and slows release of new types. A horticultural program reported to cause early flowering was tested at the Whitmore Citrus Research Foundation farm (Lake County, FL). Treatments compared on each genotype were: 1) untrained control, 2) training to a single upright shoot (TSUS), 3) TSUS with weekly thorn removal, 4) TSUS with girdling in December, 5) TSUS with weekly thorn removal and girdling (complete reported program), and 6) TSUS with a growth regulator applied in December. Trunk diameter increased faster on untrained vs. TSUS trees. In January 2010, 23 hours were <-4.4°C and one hour <-6.7°C. Trees were scored for cold damage: average rating of untrained trees was “slight defoliation” while TSUS trees averaged “complete defoliation and substantial dieback”. In control trees 14% were killed and 23% were dead or displayed dieback (DODB), while TSUS trees of treatments 2, 3 and 6 had mortality of 9-17%. Girdled trees had higher mortality at 29-43%. Percent of trees with DODB were 54%-77% in TSUS treatments. Even though a cold protection tarp system was installed and used on surviving trees during the period of coldest weather in early Dec, further damage occurred following freezing conditions in December 2010 with 17 hours <-4.4°C and one hour <-6.7°C. TSUS trees again showed much more damage than untrained trees. These techniques increase risk of serious winter-injury, and require more elaborate cold protection to be implemented in colder areas such as Lake County.

Technical Abstract:
Citrus seedling juvenility delays new hybrid evaluation, slows cultivar release, and slows introgression of new traits. A horticultural program reported to overcome citrus juvenility was tested at the Whitmore Citrus Research Foundation farm (Lake County), using replicated Hirado Buntan x Clementine seedlings and standard cultivars all propagated onto US-812 rootstock. Treatments compared on each genotype were: 1) untrained control, 2) training to a single upright shoot (TSUS), 3) TSUS with weekly thorn removal, 4) TSUS with girdling in December, 5) TSUS with weekly thorn removal and girdling (complete juvenility reduction program), and 6) TSUS with soil paclobutrazol in December. Trunk diameter increased faster on untrained vs. TSUS trees. In January 2010, 23 hours were <-4.4°C and one hour <-6.7°C. Trees were scored for cold damage: average rating of untrained trees was “slight defoliation” while TSUS trees averaged “complete defoliation and substantial dieback”. In control trees 14% were killed and 23% were dead or displayed dieback (DODB), while TSUS trees of treatments 2, 3 and 6 had mortality of 9-17%. Girdled trees had higher mortality at 29-43%. Percent of trees with DODB were 54%-77% in TSUS treatments. Even though a cold protection tarp system was installed and used on surviving trees during the period of coldest weather in early December, further damage occurred following freezing conditions in Dec 2010 with 17 hours <-4.4°C and one hour <-6.7°C. TSUS trees again showed much more damage than untrained trees. These techniques increase risk of serious winter-injury, and require more elaborate cold protection to be implemented in colder areas such as Lake County.